1. Technical Field
The present invention relates to a bulge forming method and to a bulge forming apparatus therefor, in which a raw material tube arranged between a die and a rod is formed into a shape of an inner surface of the die. The raw material tube is formed by moving the rod in the axial direction of the raw material tube while compressive stress is applied to the raw material tube and a pressurized liquid is supplied into the raw material tube. In particular, the present invention relates to improvements in sealing portions for sealing a space between the raw material tube and the rod.
2. Background Art
Bulge forming methods are used for forming shapes of containers, worked tubes, hollow structural parts of automobiles and various machines, etc. In the bulge forming method, a raw material tube is expanded by partially bulging without decreasing the wall thickness thereof, and the raw material tube is thereby formed into a predetermined shape. As a bulge formed tube obtained by this method, flexible tubes, bellows tubes, and expandable tubes, may be mentioned. The flexible tube has bellows with plural bulged portions that are flexibly bendable, the bellows tube has a larger surface area than that of an ordinary tube and is used for releasing heat, and the expandable tube has bellows portions that have the elastic characteristics of a spring.
In the bulge forming method, for example, a bulge forming apparatus 1 shown in FIG. 7 may be used (for example, see Japanese Patent Application Laid-Open No. 2001-321841). FIG. 7 is a sectional side view showing a schematic structure of a bulge forming apparatus 1 and shows a structure of an upper half portion of the bulge forming apparatus 1. In order to simplify the figures, a position of a right end of a raw material tube 2 with respect to a die 10 is the same in all of the figures. The bulge forming apparatus 1 is provided with a die 10 formed so that the raw material tube 2 is arranged inside thereof, and an abutting die 20 for abutting a left end portion of the raw material tube 2. Each of the die 10 and the abutting die 20 is made up of a pair of an upper die and a lower die. The die 10 and the abutting die 20 are fixed by fixing devices (not shown in the figure) for preventing the opening of the die 10 and the abutting die 20.
The die 10 has an inner surface having a bellows shape 11 in which plural crest portions 11A and root portions 11B are alternately formed. The crest portions 11A and the root portions 11B are, for example, periodically formed and have axially symmetrical shapes. A rod 30 is provided inside the raw material tube 2 so as to be movable along the axial direction of the raw material tube 2. A pressurized liquid supply path 31 is formed inside the rod 30 so that a pressurized liquid L is supplied therefrom through a pressurized liquid supply opening 31A to a space between the raw material tube 2 and the rod 30.
The rod 30 has a surface in the circumferential direction, on which a pair of grooves 32 and 33 is formed having the pressurized liquid supply opening 31A therebetween. The grooves 32 and 33 are mounted with ring-shaped sealing portions 40 and 50, respectively, and the sealing portions 40 and 50 seal the space between the raw material tube 20 and the rod 30. The sealing portion 50 is in the posterior side of the forming direction, and a pressurized liquid collecting path (not shown in the figure) for collecting a pressurized liquid L that has been used in forming is formed at the left of the sealing portion 50 of the rod 30A. The collected pressurized liquid L is supplied to the pressurized liquid supply path 31 and is reused. The rod 30 has a right end portion provided with a moving device (not shown in the figure) for moving the rod 30 to the right in the axial direction (in the forming direction). A raw material tube pressing device (not shown in the figure) for applying compressive stress F to the raw material tube 2 toward the abutting die 20 is formed at the right side of the die 10.
In the bulge forming apparatus 1, the left end portion of the raw material tube 2 is butted with the abutting die 20. Then, a pressurized liquid L is supplied to the space between the raw material tube 2 and the rod 30 from the pressurized liquid supply opening 31A, while compressive stress F is applied to the raw material tube 2 from the right end portion thereof to the left in the axial direction (to the posterior side of the forming direction). The pressurized liquid L is set at high pressure so that the raw material tube 2 is deformable according to the crest portions 11A of the bellows shape 11 of the die 10, and the sealing portions 40 and 50 seal the space between the raw material tube 2 and the rod 30. In this condition, the rod 30 is moved to the right in the axial direction, whereby the raw material tube 2 is bulged each time the pressurized liquid L is supplied to a space corresponding to a crest portion 11A of the die 10. The raw material tube 2 is bulged according to the shape of the crest portion 11A by the pressurized liquid L at high pressure. Thus, crest portions 2A and root portions 2B are formed at the raw material tube 2 one by one, in order, from the left side, whereby the raw material tube 2 is expanded into the shape corresponding to the bellows shape 11 of the die 10.
In the tube expansion performed by the bulge forming apparatus 1, the pressurized liquid L is set at high pressure in order to bulge the raw material tube. Therefore, the following problems occur in the sealing conditions of the sealing portions 40 and 50 each time the pressurized liquid supply opening 31A is moved between crest portions 11A of the die 10 after a first crest portion 2A is formed.
In the tube expansion, for example, after crest portions 2A are formed at the raw material tube 2 as shown in FIG. 8A, in order to form a next crest portion (third crest portion), the rod 30 is moved in a direction of a wide arrow in FIG. 8B in a condition in which the pressurized liquid L is maintained at high pressure as shown in FIG. 8B. In this case, the sealing portion 40 is in the anterior side of the forming direction and passes the third crest portion 11A having a large space between the die 10 and the rod 30. At that time, the raw material tube 2 bulges according to the shape of the third crest portion 11A, and therefore, pressing power is not sufficiently applied from the die 10 to the sealing portion 40. As a result, the sealing condition of the sealing portion 40 becomes inferior, and the pressurized liquid L may leak from the sealing portion 40 to the outside in the right side. Therefore, the liquid pressure must be increased in order to maintain the pressurized liquid L at high pressure, thereby causing wasting time.
On the other hand, when the sealing portion 50 passes the first crest portion 2A formed at the raw material tube 2 as shown in FIG. 8B, a large gap is formed between the sealing portion 50 and the first crest portion 2A, whereby the pressurized liquid L may leak from the gap to the outside in the left side. Therefore, the liquid pressure must be increased in order to maintain the pressurized liquid L at high pressure, thereby causing wasting time. The above problems in the sealing conditions of the sealing portions 40 and 50 occur each time the pressurized liquid supply opening 31A is moved between crest portions 11A of the die 10 after the first crest portion 2A is formed.
There may be a countermeasure for the problem in the sealing condition of the sealing portion 40. In this countermeasure, the liquid pressure of the pressurized liquid L is switched between high and low according to the positions of the sealing portions 40 and 50 with respect to the bellows shape 11 of the die 10.
For example, a first crest portion 2A is formed as shown in FIG. 9A. Then, the pressurized liquid L is set at low pressure so as not to deform the raw material tube 2 before the rod 30 is moved to the right in the axial direction (in the direction of a wide arrow in the figure) in order to form a next crest portion as shown in FIG. 9B. The pressurized liquid supply opening 31A is moved to the vicinity of a second crest portion 11A, and the sealing portion 40 is moved to a root portion 11B in the right side of the second crest portion 11A and is thereby maintained. Next, the movement of the rod 30 is stopped as shown in FIG. 9C, and the pressurized liquid L is set at high pressure, whereby a second crest portion 2A is formed at the raw material tube 2.
As shown in FIG. 9D, the same operation as in FIG. 9B is performed, and the liquid pressure of the pressurized liquid L is set low. The pressurized liquid supply opening 31A is moved to the vicinity of a third crest portion 11A, and the sealing portion 40 is moved to a root portion 11B in the right side of the third crest portion 11A and is thereby maintained. Such switching of the liquid pressure of the pressurized liquid L between high and low is performed each time a crest portion 2A is formed and the pressurized liquid supply opening 31A is moved between crest portions 11A. Accordingly, the problems due to the sealing condition of the sealing portion 40 do not occur.
The liquid pressure of the pressurized liquid L must be switched between high and low repeatedly in order to expand the raw material tube 2. Therefore, the switching of the liquid pressure takes time, and it is difficult to perform tube expansion at a high rate. Since the pressurized liquid L cannot be continuously maintained at high pressure, shape fixability of the raw material tube 2 with respect to the die 10 is decreased, whereby accuracy of the tube expansion is decreased, and variations among products may be increased. Specifically, in forming a bellows shape having plural crest portions 2A at the raw material tube 2, the decrease in the shape fixability has a large effect.